Adjusting eye heights and optical power levels of a multi-level optical signal

ABSTRACT

A multi-level optical signal is sampled to generate an eye diagram. The signal can be adjusted when eyes in the eye diagram have different heights. More specifically, a first value is determined, and the height of a first eye is adjusted using the first value. The first value is multiplied by a stored factor to produce a second value, and the height of a second eye is adjusted using the second value, and so on for other eyes. As a result, eye heights are the same. Similarly, optical power levels of the signal can be adjusted when the levels are not equally spaced. As a result, the optical power levels are equally spaced.

This is a continuation of U.S. patent application Ser. No. 17/402,169filed Aug. 13, 2021, issuing as U.S. Pat. No. 11,616,578 on Mar. 28,2023, which is incorporated herein in its entirety.

BACKGROUND

There is an ever-increasing demand for more bandwidth in networks thatcommunicate digital information. One way to help meet that demand is touse multi-level optical signaling. One type of multi-level opticalsignaling uses pulse-amplitude modulation (PAM), in which multipletransmission or optical power levels are used to communicate multiplebits (symbols) during each unit interval (UI) of time. For example, aPAM4 signaling scheme uses four optical power levels to transmit two-bitsymbols (e.g., 00, 01, 10, and 11) per UI.

The optical power levels are preferably equally spaced from one another,and it is also preferable that the equal spacing be maintained overtime. Linearity is a measure of the change in the spacing of the opticalpower levels, and a linearity value of one indicates perfect symmetry(spacing) between the levels over time. Generally speaking, equallyspaced and linear power levels result in a well-defined signal thatallows a receiver to more easily and accurately resolve the symbols inthe signal.

The spacing of power levels can be monitored by, for example, repeatedlysampling a multi-level signal and displaying the results on anoscilloscope, where the vertical axis of the display represents theamplitude of the signal relative to the optical power levels, and thehorizontal axis represents time. The displayed results are commonlyreferred to as an eye diagram.

FIG. 1 is an example of an eye diagram 100 for a PAM4 signal. The fouroptical power levels are labeled P0, P1, P2, and P3. The height(amplitude) of the “bottom” eye 102 is the difference between P0 and P1,the height of the “middle” eye 104 is the difference between P1 and P2,and the height of the “top” eye 106 is the difference between P2 and P3.

Multi-level optical signals are typically generated using some type oflaser or other optical signal generator that is modulated by drivers.For a PAM4 signal, there are three drivers, one driver per pair ofoptical power levels (e.g., P0 and P1) or per eye.

A characteristic of lasers is that the light output may not be a linearfunction of electrical current or voltage. For example, the amount oflight output can also be a function of operating temperature, which canvary over time. Laser slope efficiency is a measure of laser output(optical power) versus input power, and laser slope efficiency decreasesas the operating temperature increases. Also, the laser threshold (e.g.,the current at which lasing begins) is strongly affected by operatingtemperature. Therefore, generally speaking, the amount of light outputby a laser can vary over time as a function of operating temperature. Assuch, the spacing between optical power levels (e.g., P0, P1, P2, andP3) and the eye heights corresponding to those power levels can alsovary and so may not be equal, as illustrated in the example of FIG. 1 .Unequal spacing and eye heights are reflected in the numbers on theright-hand side of the eye diagram 100, which are arbitrary measures ofeye height starting from a base value of one.

In the example of FIG. 1 , the height of the top eye 106 (the spacingbetween P2 and P3) is reduced relative to the height of the middle eye104, while the height of the bottom eye 102 (the spacing between P0 andP1) is enlarged relative to the height of the middle eye. Thus, the topeye 106 requires more modulation by its respective driver so that itsheight will be the same as that of the middle eye 104. Similarly, thebottom eye 102 requires less modulation by its respective driver so thatits height will be the same as that of the middle eye 104.

The top, middle, and bottom eye heights are typically adjustedindependently of one another to achieve the desired eye height andlinearity. This entails the use of three different sets of modulationoutput swing settings to compensate for the effects of changes inoperating temperature as described above. The modulation output swingsettings are stored in lookup tables (LUTs). Because the top, middle,and bottom eye heights are adjusted independently and there are threesets of settings, three LUTs are needed. This can increase memorystorage requirements and/or computational overhead. Other conventionalschemes that are used to independently adjust the top, middle, andbottom eye heights also have these types of disadvantages.

Thus, what is needed is a method and/or device that can be used toaccurately adjust the eye heights and spacing between optical powerlevels of multi-level optical signals to maintain equal eye heights andequal spacing between optical power levels, considering that the opticaloutput of lasers can change with time and temperature.

SUMMARY

Embodiments according to the present disclosure introduce methods(processes) and devices (circuits and systems) that address thedisadvantages and satisfy the needs described above.

In embodiments, a multi-level optical signal (e.g., a multi-levelpulse-amplitude modulation (PAM) signal) is generated with a source suchas a laser or another type of optical signal generator. The multi-leveloptical signal is sampled to generate an eye diagram. The multi-leveloptical signal is adjusted when eyes in the eye diagram have differenteye heights. More specifically, a first value is determined. Forexample, the first value can be determined by selecting it from a lookuptable (LUT), or the first value can be determined based on a feedbacksignal that corresponds to a measure of the optical power of the lightgenerated by the source (referred to herein as closed-loop modulationcontrol). The height of a first eye of the multi-level optical signal isadjusted using the first value. The first value is multiplied by astored factor to produce a second value, the height of a second eye ofthe multi-level optical signal is adjusted using the second value, andso on for any other eyes. As a result, the eye heights are the same.

Optical power levels of the optical signal are similarly adjusted whenthe optical power levels are not equally spaced. More specifically, afirst value is determined (e.g., from an LUT or based on a feedbacksignal as mentioned above). The first value is multiplied by a storedfactor to produce a second value. Spacing between a first pair of theoptical power levels is adjusted using the first value, spacing betweena second pair of the optical power levels is adjusted using the secondvalue, and so on for any other pairs of optical power levels. As aresult of those adjustments, the optical power levels are equallyspaced.

In embodiments, the multi-level optical signal or PAM signal is a PAM4signal. In such embodiments, the first value can be multiplied by astored factor or factors to produce a value or values that can be usedto adjust more than two eye heights or pairs of optical power levels. Inother words, embodiments according to the present disclosure can beextended to implementations that use more than four optical powerlevels.

Embodiments according to the present disclosure thus eliminate the needfor three LUTs, replacing them with a single LUT or with closed-loopmodulation control (which does not require LUTs), thereby reducingmemory storage requirements and computational overhead while stillaccurately adjusting the eye heights and spacing between optical powerlevels of multi-level optical signals and maintaining equal eye heightsand equal spacing between optical power levels, even as the opticaloutput of a laser or other optical signal generator varies with time andtemperature.

Embodiments according to the present disclosure also simplifyimplementation of closed-loop modulation control, as the control looponly needs to adjust one eye height or one pair of optical power levelsin response to a change in laser slope efficiency, because the other eyeheights/optical power levels will be automatically scaled with thatadjustment (using the stored factors as described above) to maintainlinearity of the optical output of a laser or other optical signalgenerator.

These and other objects and advantages of the various embodiments of theinvention will be recognized by those of ordinary skill in the art afterreading the following detailed description of the embodiments that areillustrated in the various drawing figures.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, which are incorporated in and form a part ofthis specification and in which like numerals depict like elements,illustrate embodiments of the present disclosure and, together with thedetailed description, serve to explain the principles of the disclosure.

FIG. 1 is an example of an asymmetric eye diagram for a multi-leveloptical signal with unequal eye heights and uneven spacing of opticalpower levels.

FIG. 2A is a block diagram illustrating selected elements of a circuitfor modulating a source of a multi-level optical signal in embodimentsaccording to the present disclosure.

FIG. 2B is a block diagram illustrating selected elements of a circuitfor modulating a source of a multi-level optical signal in embodimentsaccording to the present disclosure.

FIG. 3 is an example of a symmetric eye diagram for a multi-leveloptical signal with equal eye heights and optical power level spacing,resulting from adjustments made using the circuits of FIGS. 2A and 2Band the methods of FIGS. 4 and 5 in embodiments according to the presentdisclosure.

FIGS. 4 and 5 are flowcharts of methods for adjusting eye height andoptical power level spacing of a multi-level optical signal inembodiments according to the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the various embodiments of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. While described in conjunction with theseembodiments, it will be understood that they are not intended to limitthe disclosure to these embodiments. On the contrary, the disclosure isintended to cover alternatives, modifications and equivalents, which maybe included within the spirit and scope of the disclosure as defined bythe appended claims. Furthermore, in the following detailed descriptionof the present disclosure, numerous specific details are set forth inorder to provide a thorough understanding of the present disclosure.However, it will be understood that the present disclosure may bepracticed without these specific details. In other instances, well-knownmethods, procedures, components, and circuits have not been described indetail so as not to unnecessarily obscure aspects of the presentdisclosure.

Some portions of the detailed descriptions that follow are presented interms of procedures, logic blocks, processing, and other symbolicrepresentations of operations on data bits. These descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. In the present application, a procedure,logic block, process, or the like, is conceived to be a self-consistentsequence of steps or instructions leading to a desired result. The stepsare those utilizing physical manipulations of physical quantities.Usually, although not necessarily, these quantities take the form ofelectrical or magnetic signals capable of being stored, transferred,combined, compared, and otherwise manipulated in a computing system. Ithas proven convenient at times, principally for reasons of common usage,to refer to these signals as transactions, bits, values, elements,symbols, characters, samples, pixels, or the like.

It should be borne in mind, however, that all of these and similar termsare to be associated with the appropriate physical quantities and aremerely convenient labels applied to these quantities. Unlessspecifically stated otherwise as apparent from the followingdiscussions, it is appreciated that throughout the present disclosure,discussions utilizing terms such as “sampling,” “receiving,”“adjusting,” “selecting,” “multiplying,” “modulating,” “reading,”“generating,” “inputting,” “providing,” or the like, refer to actionsand processes (e.g., the methods of FIGS. 4 and 5 ) of a circuit ordevice or system (e.g., the circuit or device or system of FIG. 2A orFIG. 2B).

The discussion below refers to “eye heights” and “optical power levelspacing.” It is understood that there is a relationship between thesetwo terms, and in a sense these terms may be considered to besynonymous. Thus, instances in the following disclosure in which onlyeye height is discussed may be equally applicable to optical powerlevels, and vice versa.

FIG. 2A is a block diagram illustrating selected elements of a circuit(or system) 200 for modulating a source 230 of a multi-level opticalsignal 232 in embodiments according to the present disclosure. Thecircuit 200 can include elements or components in addition to thoseillustrated and described below, and the elements or components can becoupled as shown in the figure or in a different way. Some of the blocksmay be described in terms of the function they perform. While describedand illustrated as separate blocks, the present disclosure is not solimited; that is, for example, a combination of these blocks/functionscan be integrated into a single block that performs multiple functions.

The multi-level optical signal 232 may be a pulse-amplitude modulation(PAM) signal, more specifically a Pulse-Amplitude Modulation 4-Level(PAM4) signal, and more generally a PAM signal having any number ofoptical power levels (e.g., more than four such levels).

In embodiments, the source 230 is, includes, or is coupled to a laser orother optical signal generator. In such embodiments, the source 230 canbe, but is not limited to, an electro-absorption modulated laser (EML),a directly modulated laser (DML), a Mach-Zehnder (MZ) modulated laser,or a vertical-cavity surface-emitting laser (VCSEL). In operation, thesource 230 produces the multi-level optical signal 232.

In the example of FIG. 2A, the circuit 200 is configured for a PAM4signal. However, the present disclosure is not so limited. That is, thecircuit 200 can be configured for signals that have more than fouroptical power levels, by adding one or more multipliers,digital-to-analog converters (DACs), and drivers, for example.

In the FIG. 2A embodiments, the circuit 200 includes a first DAC 202, afirst driver 204 coupled to an output of the DAC 202, a second DAC 212,a second driver 214 coupled to an output of the DAC 212, a third DAC222, and a third driver 224 coupled to an output of the DAC 222. DACsand laser drivers are known in the art.

Each of the drivers 204, 214, and 224 is coupled to the source 230. EachDAC and driver pair may be referred to herein as a device or subcircuit.In general, each DAC and driver pair modulates an input (e.g., an amountof current or voltage) to the source 230, to adjust the height of an eyeassociated with that pair or to adjust the spacing between acorresponding pair of optical power levels. A pair of optical powerlevels refers to the minimum and maximum power levels of each of levelof the multi-level optical signal (e.g., the levels P0 and P1 of FIG. 3are a pair).

In the embodiments of FIG. 2A, the circuit 200 is coupled to amicrocontroller 240 that includes, or is coupled to, a memory. In anembodiment, the memory is or includes a lookup table (LUT) 242. The LUT242 includes a number of temperature-dependent values or inputs that areused to adjust or modulate the optical signal produced by the source230, as will be described in further detail below. The values in the LUT242 may also be referred to as modulation output swing settings.

In the example configuration of FIG. 2A, a value is selected and readfrom the LUT 242, and that value (which may be referred to herein as thefirst value) is input to one of the DACs 202, 212, or 222, and is alsoinput to the multiplier 206 and to the multiplier 208. In the example ofFIG. 2A, the first value is input to the DAC 202. Thus, in this example,the first value is input to the “middle” DAC, which is associated withadjusting the height of the middle eye of an eye diagram as will bedescribed. However, embodiments according to the present disclosure arenot so limited, and the circuit 200 can be configured to input the firstvalue to the “top” DAC 212 associated with adjusting the top eye height,or to the “bottom” DAC 222 associated with adjusting the bottom eyeheight, depending on the configuration of the circuit 200.

In the example configuration of FIG. 2A, the multiplier 206 multipliesthe selected (first) value by a predetermined and stored factor or ratioto produce a second value. Similarly, the multiplier 208 multiplies thefirst value by a predetermined and stored factor or ratio to produce athird value. The factor or ratio used by the multiplier 206, and thefactor or ratio used by the multiplier 208, may have the same value orthey may have different values. The value of each factor is determinedas described below. The factor(s) may be stored in a register orregisters of, for example, the microcontroller 240.

In essence, in the example of FIG. 2A, the factor used by the multiplier206 is the ratio of the middle eye height to the top eye height whendriven equally, and the factor used by the multiplier 208 is the ratioof the middle eye height to the bottom eye height when driven equally.Under ideal conditions, each ratio would have a value of one.

The factor or ratio used by the multiplier 206, and the factor or ratioused by the multiplier 208, can be determined and stored during setup orcalibration of the circuit 200 and source 230, for example. The factoror factors can be determined by modulating the source 230 over a rangeof operating temperatures to determine temperature-dependent modulationoutput swing settings that achieve linearity for one of the eyes (e.g.,the middle eye) of the multi-level optical signal 232 over time andtemperature. Those settings/factors are stored in the LUT 242 as afunction of temperature. Then, the other eyes (e.g., the top and bottomeyes) are scaled so that their eye heights are equal to the eye heightof the middle eye. The amount of scaling per eye establishes the factoror factors used by the multipliers 206 and 208.

In operation, the DAC 202 receives the first value read from the LUT 242and outputs an analog signal to the driver 204, and the driver 204modulates the source 230 using the output of the DAC 202. The DAC 212receives the second value from the multiplier 206 and outputs an analogsignal to the driver 214, and the driver 214 modulates the source 230using the output of the DAC 212. Similarly, the DAC 222 receives thethird value from the multiplier 208 and outputs an analog signal to thedriver 224, and the driver 224 modulates the source 230 using the outputof the DAC 222. The drivers 204, 214, and 224 can modulate the source230 based on current or on voltage.

FIG. 2B is a block diagram illustrating selected elements of the circuit200 for modulating a source of a multi-level optical signal withclosed-loop modulation control in embodiments according to the presentdisclosure. In the embodiments of FIG. 2B, the circuit 200 is coupled toa microcontroller or modulation control state machine 250 (hereinafter,the microcontroller 250).

In the FIG. 2B embodiments, a photodetector (e.g., a monitor photodiode234) receives light generated by the source 230 and generates a feedbacksignal that is a measure of the optical power of the light. That is, thecurrent or voltage level of the feedback signal corresponds to theoptical power of the optical signal. The feedback signal can beconverted to a digital signal by an analog-to-digital converter (notshown), and input to the microcontroller 250. The microcontroller 250can compare the measure of optical power represented by the feedbacksignal to a stored value, and can determine the first value by adjustingthe value that is being input to the DAC 202, based on the feedbacksignal (e.g., based on the difference between the measured value and thestored value). In general, with closed-loop modulation control of alaser, laser slope efficiency is sensed, and the modulation current orvoltage is adjusted accordingly.

Once the first value is determined, the circuit of FIG. 2B functionslike the circuit of FIG. 2A, at least to the extent described herein.Thus, embodiments according to the present disclosure can be used withan approach based on LUTs as described in conjunction with FIG. 2A, orwith an approach based on closed-loop modulation control as described inconjunction with FIG. 2B. More generally speaking, embodiments accordingto the present disclosure can be used with compatible approaches fordetermining the first value, in addition to those described herein.

In the example of FIG. 2B, the microcontroller 250 is shown as being thesource of the factors used by the multipliers 206 and 208; however, thepresent disclosure is not so limited, and those factors can be providedfrom another source (e.g., another microcontroller).

Thus, in embodiments according to the present disclosure, modulationoutput swing settings are needed for only a reference eye, because thesettings for the other eyes are multiples of the settings for thereference eye. As such, only a single LUT is needed to store themodulation output swing settings, instead of the three LUTs needed inthe conventional art. With closed-loop modulation control, no LUTs areneeded. Consequently, embodiments according to the present disclosurereduce memory storage requirements, while still accurately adjusting theeye heights (and spacing between optical power levels) of multi-leveloptical signals and maintaining equal eye heights (and equal spacingbetween optical power levels) even as the optical output of a laservaries with time and temperature.

Also, setup and calibration are simplified, as only a single set ofmodulation output swing settings need to be written to and stored in theLUT 242. Furthermore, computational overhead during operation isreduced, because only the selected setting (the first value) needs to bewritten to a DAC, and it is written to only a single DAC (e.g., the DAC202). In addition, operation of the circuit 200 is simplified, becauseit is not necessary to synchronize the DACs 202, 212, and 222; instead,the second and third values determined by the multipliers 206 and 208are calculated automatically and input to the DACs 212 and 222 when thefirst value is determined and input to the DAC 202. Even if theaforementioned ratios are each equal to one, the disclosed inventionstill provides the above advantages.

In the discussion to follow, the middle eye (and associated opticalpower levels) may be described as the reference for adjusting theheights of the top and bottom eyes (and their associated optical powerlevels). However, as already noted, the present disclosure is not solimited, and any one of the other eyes can be chosen to be the referenceat the time of setup and calibration and during subsequent operation, aslong as the circuit 200 is configured accordingly.

FIG. 3 is an example of an eye diagram 300 for a multi-level opticalsignal with equal eye heights and equal spacing of optical power levels,as a result of adjustments made according to the present disclosure,using the embodiments of FIGS. 2A and 2B, and the methods of FIGS. 4 and5 . The eye diagram 300 is for a PAM4 signal, and hence includes fouroptical power levels (P0, P1, P2, and P3) that define a bottom eye 302,a middle eye 304, and a top eye 306.

The numbers on the right-hand side of the eye diagram 300 aredimensionless measures of eye height and optical power level startingfrom a base value of one, demonstrating both equal heights of the eyes302, 304, and 306 and equal spacing between the optical power levels P0,P1, P2, and P3.

FIGS. 4 and 5 are flowcharts 400 and 500 of methods for adjusting amulti-level optical signal in embodiments according to the presentdisclosure. All or some of the operations represented by the blocks inthose flowcharts can be executed by, for example, the elements of FIG.2A or of FIG. 2B. Methods for adjusting a multi-level optical signalaccording to the present disclosure can include operations in additionto those shown in the figures and described below. Also, the operationsshown in the figures and described below may be performed in a differentorder than that presented.

In block 402 of FIG. 4 , a multi-level optical signal (e.g., amulti-level PAM signal such as but not limited to a PAM4 signal) isgenerated with the source 230. In block 404, the multi-level signal issampled to generate an eye diagram. In block 406, the multi-level signalis adjusted when eyes in the eye diagram have different eye heights.

In block 406 a, a first value is determined (e.g., from the LUT 242 orwith closed-loop modulation control). In block 406 b, the height of afirst eye (the middle eye in the examples of FIGS. 2A and 2B) of themulti-level optical signal is adjusted using the first value. Morespecifically, the first value is input to the DAC 202, the output of theDAC 202 is provided to the driver 204, and the driver 204 modulates aninput (e.g., current or voltage) to the source 230.

In block 406 c, the first value is multiplied (e.g., by the multiplier206) by a stored factor to produce a second value. In block 406 d, theheight of a second eye (the top eye in the examples of FIGS. 2A and 2B)of the multi-level optical signal is adjusted using the second value.More specifically, the second value is input to the DAC 212, the outputof the DAC 212 is provided to the driver 214, and the driver 214modulates an input (e.g., current or voltage) to the source 230.

Blocks 406 c and 406 d can be repeated for a third eye (the bottom eyein the examples of FIGS. 2A and 2B) using the multiplier 208, a thirdvalue produced by that multiplier, the DAC 222, and the driver 224. Ingeneral, blocks 406 c and 406 d can be repeated for any practical numberof eyes.

As a result of the operations just described, the height of the eyes inthe eye diagram are equal or substantially equal.

In block 502 of FIG. 5 , a multi-level optical signal (e.g., amulti-level PAM signal such as but not limited to a PAM4 signal) isgenerated with the source 230. In block 504, optical power levelsproduced by the source 230 are measured. Techniques for measuring outputoptical power are known in the art. In block 506, optical power levelsof the optical signal are adjusted when the optical power levels are notequally spaced.

In block 506 a, a first value is determined (e.g., from the LUT 242 orwith closed-loop modulation control). In block 506 b, the first value ismultiplied (e.g., by the multiplier 206) by a stored factor to produce asecond value.

In block 506 c, spacing between a first pair of the optical power levelsis adjusted using the first value. More specifically, the first value isinput to the DAC 202, the output of the DAC 202 is provided to thedriver 204, and the driver 204 modulates an input (e.g., current orvoltage) to the source 230. In the examples of FIGS. 2A and 2B, thefirst pair of the optical power levels includes the levels associatedwith the middle eye (e.g., P1 and P2 of FIG. 3 ).

In block 506 d, spacing between a second pair of the optical powerlevels is adjusted using the second value. In the examples of FIGS. 2Aand 2B, the second pair of the optical power levels includes the levelsassociated with the top eye (e.g., P2 and P3 of FIG. 3 ). Morespecifically, the second value is input to the DAC 212, the output ofthe DAC 212 is provided to the driver 214, and the driver 214 modulatesan input (e.g., current or voltage) to the source 230.

Blocks 506 b and 506 d can be repeated for a third pair of optical powerlevels (those associated with the bottom eye in the examples of FIGS. 2Aand 2B; e.g., P0 and P1 of FIG. 3 ) using the multiplier 208, a thirdvalue produced by that multiplier, the DAC 222, and the driver 224. Ingeneral, blocks 506 b and 506 d can be repeated for any practical numberof pairs of optical power levels.

As a result of the operations just described, the spacing betweenoptical power levels is equal or substantially equal.

While the foregoing disclosure sets forth various embodiments usingspecific block diagrams, flowcharts, and examples, each block diagramcomponent, flowchart step, operation, and/or component described and/orillustrated herein may be implemented, individually and/or collectively,using a wide range of configurations. In addition, any disclosure ofcomponents contained within other components should be considered asexamples because many other architectures can be implemented to achievethe same functionality.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in this disclosure is not necessarilylimited to the specific features or acts described above. Rather, thespecific features and acts described above are disclosed as exampleforms of implementing this disclosure.

Embodiments according to the invention are thus described. While thepresent invention has been described in particular embodiments, theinvention should not be construed as limited by such embodiments, butrather construed according to the following claims.

What is claimed is:
 1. A circuit comprising: a plurality of pulseamplitude modulation drivers configured to receive different inputsignal pairs and corresponding ones of a plurality of drive values, andgenerate corresponding ones of a plurality of driven input signal pairs,wherein the plurality of drive values includes a first drive value andadditional ones of the plurality of drive values that are a function ofthe first drive value and corresponding ones of a number of drivefactors; and a source, coupled to the plurality of pulse amplitudemodulation drivers, and configure to generate an output optical signalas a function of the plurality of driven input signal pairs, wherein theoptical signal output varies over time as a function of an operatingtemperature of the source and the plurality of drive values areconfigured to maintain linearity of eye height values of the outputoptical signal.
 2. The circuit of claim 1, wherein the optical signaloutput comprises a Pulse-Amplitude Modulation 4-Level (PAM4) signal. 3.The circuit of claim 1, the source is selected from a group consistingof an electro-absorption modulated laser; a directly modulated laser; aMach-Zehnder modulated laser; and a vertical-cavity surface-emittinglaser.
 4. The circuit of claim 1, wherein generating the output opticalsignal as a function of the plurality of drive input signal pairscomprises the plurality of pulse amplitude modulation drivers generatingcorresponding ones of the plurality of driven input signals thatmodulate a current input to the source.
 5. The circuit of claim 1,wherein generating the output signal as a function of the plurality ofdrive input signal pairs comprises the plurality of pulse amplitudemodulation drivers generating corresponding ones of the plurality ofdriven input signals that modulate a voltage input to the source.
 6. Thecircuit of claim 1, wherein the first drive value is selected from alookup table comprising a plurality of temperature-dependent valuesincluding the first value.
 7. A circuit comprising: a controllerconfigured to receive a feedback signal, and to generate a first drivevalue of a plurality of drive values and a number of drive factors; anumber of multipliers configured to generate corresponding additionaldrive values of the plurality of drive values as a function ofcorresponding ones of the number of drive factors and the first drivevalue; a plurality of drivers configured to receive corresponding inputsignal pairs and corresponding drive values of the plurality of drivevalues, and to generate a plurality of corresponding driven input signalpairs; and an optical signal source configured to receive the pluralityof driven input signal pairs, and generate the output optical signal,wherein the plurality of drive values are configured to maintainlinearity of power levels of the output optical signal.
 8. The circuitof claim 7, wherein the output optical signal comprises aPulse-Amplitude Modulation (PAM) signal.
 9. The circuit of claim 7,wherein the output optical signal comprises a pulse-Amplitude Modulation4-Level (PAM4) signal.
 10. The circuit of claim 7, wherein the pluralityof drive values are configured to adjust optical power level spacingbetween the plurality of driven input signal pairs.
 11. The circuit ofclaim 7, wherein the feedback signal comprises a temperature measurementof the optical signal source; and the different corresponding drivesignals are generated based on the temperature measurement of theoptical signal source and a look-up table.
 12. The circuit of claim 7,wherein the feedback signal comprises an optical power output of theoptical signal source; and the different corresponding drive signals aregenerated based on the optical power output of the optical signalsource.
 13. The circuit of claim 12, wherein the controller generatesthe different corresponding drive signals based on the optical poweroutput of the optical signal source using a lookup table comprising aplurality of temperature-dependent values including the first drivevalue.
 14. The circuit of claim 7, further comprising: a plurality ofdigital-to-analog converters (DACs) configured to convert correspondingones of the plurality of drive values from digital to analog for inputto corresponding ones of the plurality of drivers.
 15. The circuit ofclaim 7, wherein the plurality of drivers modulate a current ofcorresponding driven input signal pairs to maintain linearity of powerlevels of the output optical signal.
 16. The circuit of claim 7, whereinthe plurality of drivers modulate a voltage of corresponding driveninput signal pairs to maintain linearity of power levels of the outputoptical signal.
 17. The circuit of claim 7, wherein the source isselected from a group consisting of an electro-absorption modulatedlaser; a directly modulated laser; a Mach-Zehnder modulated laser; and avertical-cavity surface-emitting laser.
 18. A method comprising:generating a first drive value of a plurality of drive values and anumber of drive factors based on a received feedback signal of anoptical signal; generating corresponding additional drive values of theplurality drive values as a function of corresponding ones of the numberof drive factors and the first drive value; generating a plurality ofcorresponding driven input signal pairs based on received correspondinginput signal pairs and corresponding drive values of the plurality ofdrive values; and generating the optical signal based on the pluralityof driven input signal pairs, wherein the plurality of drive values areconfigured to maintain linearity of power levels of the optical signal.19. The method of claim 18, wherein the optical signal comprises aPulse-Amplitude Modulation (PAM) signal.
 20. The method of claim 18,wherein the multi-level optical signal is generated by a source selectedfrom a group consisting of an electro-absorption modulated laser; adirectly modulated laser; a Mach-Zehnder modulated laser; and avertical-cavity surface-emitting laser.